In recent years an inverter employing a pulse width modulation method (hereinafter referred to simply as “PWM method”) has been widely used for driving motors. An inverter-driving with the PWM method encounters an electric potential difference (hereinafter referred to simply as “shaft voltage”) between an outer ring and an inner ring of the bearing because the electric potential at a neutral point of the winding does not stay at 0 (zero). The shaft voltage contains high-frequency component produced by switching action, and when the shaft voltage reaches a breakdown voltage of oil film formed inside the bearing, micro-electric current runs inside the bearing, thereby producing electric erosion inside the bearing. A progress of the electric erosion will encounter wavy-abrasion on the outer ring, inner ring, or balls of the bearing, and resultantly produces an abnormal sound that is one of chief causes of malfunctions of motors.
A power supply circuit of a driving circuit (including a control circuit) for inverter-driving the foregoing motor with the PWM method is electrically insulated from a primary side circuit of the power supply circuit, and from a grounding to the earth of the primary side circuit.
To suppress the electric erosion, the following measures have been taken: (1) The inner ring and the outer ring of the bearing are put in a conductive state. (2) The inner ring and the outer ring of the bearing are put in an insulated state. (3) The shaft voltage is lowered.
To be more specific about item (1), conductive lubricant is used for the bearing. However, the conductive lubricant has such drawbacks as its conductivity lowers with the passage of time, and sliding reliability is insufficient. There is another method to generate the conductive state, i.e. a rotary shaft is provided with a brush. This method has also problems such as abrasion powders are produced, and a space for the brush is needed.
To be more specific about item (2), iron balls disposed inside the bearing are replaced with non-conductive ceramic balls. Although this method can suppress substantially the electric erosion, it costs a lot, so that this method cannot be employed for generalized motors.
To be more specific about item (3), a stator iron core is electrically shorted with a metal conductive bracket for changing an electrostatic capacity, thereby lowering the shaft voltage. This method has been known to the public (e.g. refer to Patent Literature 1). There are many other methods, e.g. the stator iron of a motor is grounded (e.g. refer to Patent Literature 2), for suppressing the electric erosion of the motor bearing.
In recent years, a highly reliable mold-motor has been proposed. This mold-motor employs stator members, such as a stator iron core, which are molded by using moldable materials. In this motor, the bearing is rigidly mounted with the forgoing insulating molded members replacing metal brackets for reducing a needless high-frequency electric voltage that is produced on the outer ring side of the bearing, or for reducing a needless high-frequency electric current flowing between the outer ring and the inner ring of the bearing. However, these molded members are made of resin and have strength not enough to rigidly mount the bearing to the motor, or have dimensional accuracy not accurate enough due to molding the resin. These drawbacks tend to encounter a creep failure in the bearing. In other words, the bearing like a ball bearing, in general, has a space between its outer ring and an inner wall of the housing, in this case, transmission load produces force on the shaft in radial direction, and this force tends to produce a slipping phenomenon due to relative difference in the radial direction. This slipping phenomenon is referred to as creep, which can be usually restrained by fixing the outer ring strongly to the housing, e.g. bracket. In recent years, the motor capable of outputting a greater power has prevailed, so that the bearing needs to be fixed to the housing more strongly. This market trend inevitably entails to take measures against the creep by employing a metal bracket for fixing the bearing. This metal bracket is made of stainless steel plate and excellent in dimensional accuracy. The rotary shaft is usually supported by the bearings at two places, so that it is preferable to fix the two bearings with metal brackets because of the strength described hereinbefore and easiness of implementation.
A motor, of which rotor is provided with a dielectric layer between the shaft and the outer wall of a rotary body, is disclosed (e.g. refer to Patent Literature 3). The structure of this motor allows suppressing the electric erosion in the bearing.
Several methods have been proposed hereinbefore, but yet those conventional methods encounter the following problems:
The method disclosed in Patent Literature 1: Since this method employs a short-circuit, the voltage balance between the outer ring and the inner ring of the bearing tends to get out of balance, and the shaft voltage thus sometimes increases.
Another conventional method is this: A power supply circuit of the driving circuit (including a control circuit) for inverter-driving a motor with the PWM method is electrically insulated from a primary side circuit of the power supply circuit, and from a grounding to the earth of the primary side circuit. This structure allows excluding an electric shock, so that safety can be assured. As Patent Literature 2 discloses, a stator iron core of a motor is electrically grounded, and this structure is combined with the foregoing structure in order to suppress the electric erosion. However, this combined structure has another problem from the viewpoint of specification and characteristics of the motor, so that implementation of this combined structure is difficult.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2007-159302    Patent Literature 2: Unexamined Japanese Patent Application Publication No. 2004-229429    Patent Literature 3: International Publication No. 2009/113311